Catalytic reforming process in oil refinery processes leads to byproducts of hydrocarbon mixtures containing C8+ aromatics fractions. Such streams are generally fractionated to C8 aromatic cut, C9 aromatics cut and so on. Each of the fractions are processed to recover value added products, for example C8 cut is processed for recovery of para-xylene, C9-C12 cut finds application for either separation of specific valuable component like naphthalene, or fractioned for use as solvent, or blending stock in gasoline or diesel or fuel oil etc. The presence of oxygenates in the C8-C12 stream usually reduces the calorific value of the stream for uses such as fuel for combustion. The reduction in calorific value from the hydrocarbon to its corresponding oxygenates, (c.a. toluene to benzaldehyde, ethylbenzene to acetophenone etc.,) as disclosed by Dimitri Konovalov, in the article titled “The Calorific Value of Carbon Compounds”, JCS rans. 1923, 123, 2184-2202, is provided herein below.
CorrespondingHydrocarbonQ (Kcal/Mol)oxygenateQ (Kcal/Mol)Toluene934.4 ——————Benzaldehyde842.1Xylene1092 ——————Acetophenone1001.9
In any of these applications, it is always important to avoid formation of oxygenated compounds through auto-oxidation. The side-chains in these C8-C12 aromatics are always susceptible for auto-oxidation on exposure to air or oxygen and the propensity of auto-oxidation enhances with increase in number of carbon atoms in the side-chain. Thus it becomes an unavoidable necessity to preserve all these streams with elaborate arrangement of nitrogen blanketing in order to prevent exposure to air or oxygen. Therefore, it would be of significant industrial importance to develop an adsorptive method to remove or reduce the trace concentration of auto-oxidation products from these streams by using the suitable novel adsorbent compositions and adsorption processes.
To illustrate, further, the C8 cut, obtained in the refining or petrochemical processes, as described earlier, is meant for recovery of para-xylene and should be free from any oxygenated compound like aceto-phenone. Another illustration could be the C10 aromatic stream containing diethyl benzenes. A mixture of diethyl benzenes (obtained as by-product in styrene manufacture from ethyl benzene), are sometimes employed for recovery of para-diethyl benzene using a high pressure simulated moving bed adsorptive process using specific adsorbent, (a process developed by M/s Universal oil Products, USA), wherein it is of utmost importance that the feed stock diethyl benzene mixture should be essentially free from oxygenates or the auto-oxidation products of diethyl benzene, namely 4-ethyl benzaldehde, 4-ethyl acetophenone, 1,4 benzaldehyde, 4-acetyl acetophenone etc. It also equally holds good for the para-diethyl benzene product, a de-sorbent material, which finds use for separation of paraxylene through the well known “PAREX” process, wherein the stringent specification of the auto-oxidation products are very important. Normally the de-sorbent (para-di-ethyl benzene should essentially be free from auto-oxidation products, typically less than 1 ppm). It is to be born in mind that the same specification would be applied to the para-di-ethyl benzene (the desorbent in “PAREX” process), irrespective of the process or method of manufacturing the de-sorbent, either through disproportionation of ethyl benzene, or ethylation of ethyl benzene. Alternatively, the para-diethyl benzene could also be produced from non-conventional aromatics stream such as Mixed Xylene Solvent, containing predominantly a mixture of ethyl benzene and xylenes as described in the U.S. Pat. No. 7,709,692. However, as mentioned earlier, in any case, the desorbent (para-diehtyl benzene) should be essentially free from auto-oxidation product and the other oxygenates.
There have been several different adsorption schemes proposed for removal of oxygenated hydrocarbons but mostly are paraffinic in nature. For example, U.S. Pat. No. 6,111,162 discloses that hydrocarbons with 3 to 8 carbon atoms are treated for removal of oxygenated contaminants by an adsorbent comprising silica gel.
U.S. Pat. Nos. 7,576,248 and 7,102,044 discloses a process for removal of one or more oxygenates from C10-C15 olefin rich paraffin streams by passing the stream through an adsorbent bed comprising activated alumina, silica gel and sodium X zeolites.
U.S. Pat. No. 5,427,689 discloses how a variety of polar substances, including water, alcohols, ethers, aldehydes, ketones, amines, mercaptans, organic sulfides and carboxylic acids are removed from a hydrocarbon containing 1 to 10 carbon atoms using an absorbent composition comprising aluminum borate and zirconium borate.
Though, various processes, mainly involving the use of adsorbents for the removal of the oxygenates from the paraffinic hydrocarbon streams have been widely reported, processes for removal of the carbonyls from C8-C12 aromatic streams in general and from C10 aromatics in particular, and more specifically from para-diethyl benzene, by using adsorbents have not been reported.
The existing methods for the removal of carbonyl from aromatic streams of the kind mentioned above comprises of classical chemical method of treatment with reducing agent such as lithium aluminium hydride or sodium borohydride. Such methods consists of firstly treating the said stream with said reducing agent in the presence of methanol, followed by decomposing the extra amount of the reducing agent by addition of water and finally washing the stream to remove any excess alkali or reducing agent. Alternatively, the carbonyls could be destroyed by reducing with a metal and acid e.g. zinc and hydrochloric acid, followed by washing to remove acid/byproducts etc. Clearly, in such methods not only a large amount classical chemicals are used, but also a huge quantity of waste water is generated which is required to be further treated in an effluent treatment unit before disposal.
Another approach to remove particularly carbonyls from the said stream is solvent extraction using polar solvents such as methanol, ethanol, propanol, butanol etc, wherein the carbonyls are dissolved in the polar solvent and thereafter the solvents are separated by any known means such as settling and separation of layers, distillation etc.
Such methods have disadvantages such as the use of massive amount of solvents which is required to be purified before recycling through distillation, which in turn becomes both expensive (in terms of fixed and operating costs) as well as energy intensive.
In view of the above, there is therefore a need for a process for removal carbonyls from the aromatic hydrocarbon stream that overcomes the disadvantages associated with the classical chemical methods of the prior art.